Switchable high-pass filter arrangements are known which are constructed as electrical circuits by means of resistors and capacitors and which may be designed as simple RC elements. Such RC elements have a capacitor in series with an input and a resistor in parallel with an output of the arrangement. The high-pass filter behavior of such arrangements is described by a lower limiting frequency determined by the values of the resistor and of the capacitor. Signals above said limiting frequency which are present at the high-pass filter arrangement are not substantially influenced and are transmitted unchanged, but signals below the limiting frequency are attenuated and thus largely suppressed.
For switching of such high-pass filter arrangements, the resistor of the circuit may be designed to be switchable by means of changeover switches that are produced for example by means of field effect transistors using MOS technology. What is disadvantageous about such arrangements is that the changeover switches have parasitic capacitances, in particular with the use of field effect transistors using MOS technology. The parasitic capacitances act in series with the resistor, bridge the changeover switch for radiofrequency signals and thus form together with the resistor a parasitic low-pass filter arrangement that disturbs the behavior of the high-pass filter arrangement. Such arrangements are therefore unsuitable for radiofrequency applications.
FIG. 1 shows a switchable high-pass filter arrangement known in accordance with the prior art, which arrangement has parallel-connected resistors 40, 40′ and a capacitor 30. The arrangement filters a signal that is present at the input 10 and is transmitted to the output 20, in such a way that the high-frequency components of the signal are not influenced, but the low-frequency components are attenuated.
Situated in series with the resistor 40 is a changeover switch 50, which is switched by means of a switching signal 60 and DC-isolates the resistor 40 from the circuit and thus renders it inactive. As a result of the changeover of the switch 50, the resistance that takes effect in the signal path is altered and the limiting frequency of the arrangement is thus shifted. The changeover switch 50 may be realized by using, for example, field effect transistors produced using MOS technology. Since the changeover switch 50 is arranged in series with the resistor 40, the transistor components of the changeover switch 50, which have a parasitic capacitance 50′, act in series with the resistor 40, bridge the changeover switch 50 for radiofrequency signals and form with the resistor a separate parasitic RC element that has a low-pass filter characteristic and adversely influences the behavior of the high-pass filter arrangement.
Therefore, there is a need for switchable high-pass filter arrangements, in particular for optical receivers, which avoid the parasitic effects of the changeover switches, are suitable for radiofrequency applications, have low losses and can be produced in a simple manner.